I was directed to a source by another APUGger:
I mix the 5 liters of Xtol and put 1 liter in a bottle as working stock and the remaining 4 liters in one of these. Then I replenish from the water box. Ships flat and easy to put together. The valve works well and seals properly. I don't know if the Datacube is still available.
I don't know where you are, but I've heard that some Starbucks will sell an empty similar container, and I have picked up a similar one at Dunkin Donuts after convincing them that I wasn't going to sell something in it that I was pretending was their coffee. The survivalsolutions box is more suited to darkroom storage and use, as it can sit on a shelf with the valve placed properly for use. You have to tilt over the Dunkin Donuts box and pour, which is more likely to let air into the bladder.